If you're writing a fanfic, then it's easy: You can just use an established character as the basis for comparison. ("Phineas looked up at the teenager, who was almost a head taller than he," "Her breasts seemed enormous, more than twice the size of Vanessa's").
More generally speaking, throwing numbers at readers is probably less effective than describing the character's appearance as others might perceive it. ("She looked like an Olympic volleyball player--six feet tall, willowy, and deeply tanned." "The man's short, wide physique reminded Matt of an airlock door.") IMO, adding these subjective details makes the character's appearance more memorable for readers.